1. FIELD OF THE INVENTION
This application relates to anti-treeing additives effective in polyolefin polymers. The application further relates to improved electrical insulation materials, and improved electrical cables.
This invention further relates to an electrical cable comprising a primary insulating layer manufactured from the polyethylene and the anti-treeing additive.
2. DESCRIPTION OF THE PRIOR ART
Polymeric compositions are well-known and are used extensively as primary insulation materials for wire and cable. As an insulator, it is important the composition have various physical and electrical properties, such as resistance to mechanical cut through; stress crack resistance; and dielectric failure. Recent publications have indicated water tree growth and electrical tree growth in the primary insulation are particularly important problems since they are associated with, though not necessarily totally responsible for, dielectric failure.
An important application for a primary insulating material is in high voltage transmission and distribution cable, especially useful in direct buried underground service. Unfortunately, the efficient use of polymeric compositions in high voltage cables is precluded by a degradation process called "treeing". Treeing is an electrical pre-breakdown process. The name is given to the damage in a solid dielectric exposed to electrical stress wherein the damage visually resembles trees. Treeing can occur and progresses as a result of partial discharges or without discharges in the presence of moisture, and with impulses, ac, or dc voltages.
It is generally believed two different types of trees exist. Trees which form in the presence of water, and in particular at low voltages, are called water or electrochemical trees. When water is absent, the trees which form are called electrical trees.
Although there are many theories concerning the initiation and growth of trees, there is virtual unanimity in the belief they start at an imperfection in the cable. This imperfection can be a small void or a piece of solid contamination.
Several organic additives have been discovered which are quite effective in retarding the growth of both types of trees. Acetophenone is perhaps one of the best known anti-treeing agents in existence. It is a product of the decomposition of dicumylperoxide which has found wide use as a curing agent to produce crosslinked polyethylene. The initial decreased treeing tendency of crosslinked polyethylene is a direct result of the existence of acetophenone in the former. Unfortunately, the effect is only temporary because the acetophenone diffused out of the polyethylene with time; and the polymer's resistance to treeing becomes essentially the same as uncrosslinked polyethylene.
The prevention of treeing has also been attempted by preparing super clean resin. The inclusion of fillers or decreasing or eliminating the cable's exposure to steam during crosslinking is also helpful.
Silicones have found limited use in the area of anti-treeing. Kato, et al. (U.S. Pat. No. 3,956,420) discloses the use of a combination of ferrocene, in 8-substituted quinoline, and a silicone liquid to increase the dielectric strength of polyethylene and its voltage endurance in water. Ashcraft, et al. (U.S. Pat. No. 4,144,202) inhibits water treeing in ethylene polymer compositions by employing organosilanes containing an epoxy radical. Ashcraft, et al. (U.S. Pat. No. 4,263,158) further discloses the use of organosilanes containing C.dbd.N bonds to inhibit water treeing in ethylene polymers. Ashcraft et al. (Canadian Pat. No. 1,103,915) further discloses the use of organosilanes containing C.dbd.O bonds to inhibit water treeing in ethylene polymers.
German Offenlegungsschrift Number 2,737,430 and U.S. Pat. No. 4,299,713 disclose the addition of trialkoxysilanes to polyolefin insulation to prevent water tree formation. U.S. Pat. No. 4,332,957 discloses the use of phenoxyalkoxy-substituted silanes as water tree and electrical tree retardant additives. British Pat. No. 1,248,256, and British Pat. No. 1,277,378 disclose treating mineral fillers with organosilanes and then adding them to the polymer to decrease the porosity of the composition. Japanese Patent Number Sho 50[1981]-92946 discloses the use of silicone grafted polyolefins in combination with propionates to inhibit water treeing. Japanese Patent Number Sho 56[1981]-109404 discloses the use of diorganopolysiloxanes having a viscosity range of 30 to 500 centistokes to inhibit water treeing. This patent further discloses siloxanes modified with alkoxy groups have little effect upon water treeing.
As is evidenced by the prior art, treeing can be inhibited in two different ways. If the voids in the plastic are filled, there is slight improvement in resistance to treeing. If voltage stabilizers, such as acetophenone, are included in the polyethylene, the stabilizers are thought to trap and deactivate electrons, and thus inhibit treeing. Most, if not all, of the voltage stabilizers are mobile aromatic compounds. The mobility of the compound, however, can not be so great that it does not stay in the plastic. If the additive is too mobile and low in molecular weight, it migrates to the surface, it evaporates, and its effectiveness is totally lost.
As evidenced by the data in the present application, it is theorized that the ideal composition should contain an additive which is mobile and sufficiently compatible (soluble) with the plastic so it can migrate to the voids and solid impurities which are the points of treeing initiation. By filling and surrounding these faults in the plastic, it retards the initiation of the trees; and by filling the tree channel as it is formed, it retards the growth of the trees. If the additive can fill the tree void and consume or remove the water in the void, the additive would be additionally effective. At the same time, the additive must be sufficiently nonvolatile to assure it stays in the plastic and does not evaporate.
It is a futher object of this invention to provide a composition comprising a polyolefin and a silane anti-treeing additive; the additive being mobile, nonvolatile, and somewhat compatible (soluble) with the plastic. It is a further object of this invention to provide a cable which is manufactured from the compositions of this invention. It is a further object of the invention to provide a method for restoring reliability to underground distribution cable.